1. Field of the Invention
The invention relates generally to the field of optics and more particularly to optical add-drop multiplexers.
2. Description of Related Art
Transparent optical shared protection rings enhance wavelength division multiplexing (DWDM) networks. One conventional architecture is a two-fiber Optical Channel (OCh) shared protection ring which is a type of network protection that resembles many of the features present in current SONET bidirectional line switched rings, or SDH shared protection rings. Several OCh shared protection rings employ switching at the “end-nodes” where individual Optical Channels are added and dropped from the ring. This style of ring switching is termed “end-node” switching, as described with respect to FIGS. 1A and 1B.
Under normal conditions as depicted in FIG. 1A, an outgoing primary traffic 130 (traffic to be protected) is added at a node B 150, transported clockwise within a DWDM signal, and dropped at a node D 170 on a channel labeled λ1 110. The return traffic is added at the node D 170, transported counter-clockwise within a DWDM signal, and dropped at the node B 150 on a channel labeled λ2 120. A bidirectional primary traffic 140 is on different wavelengths in order for the protection mechanism to work without optical-to-electrical-to-optical (OEO) conversion. The labels “λ1” 110 and “λ2” 120 are generically meant to convey different wavelengths. A self-healing ring that avoids costly OEO transitions is termed optically transparent.
Under failure conditions (e.g. cable cut between the nodes B 150 and C 160) as shown in FIG. 1B, the end-nodes B 150 and D 170 bridge (make an optical copy of) their add signals and send them in both directions around the ring. End-nodes B 150 and D 170 then switch (select) their drop signals based on the best signal that arrives. This implementation is termed OCh shared protection with end-node switching. It resembles many of the features present in transoceanic SDH shared protection rings. A failure such as fiber cut may affect several Optical Channels (wavelengths) at same time. Nevertheless, failure detection, triggering, and ring switching are all completed separately for each Optical Channel. This allows a ring node to employ protection switching on some Optical Channels, and not on others.
To operate correctly, the nodes on the ring exchange automatic protection switching (APS) messages among themselves. One way to exchange messages is to place them within an Optical Supervisory Channel (OSC) signal. Typically, OSC signals are transmitted between adjacent nodes, and are separate in wavelength from the DWDM signals.
Another approach employing a transparent OCh shared protection is shown in FIGS. 2A and 2B. The primary traffic 130 is again on different wavelengths, as in FIGS. 1A and 1B. When a failure such as a cable cut 210 occurs, the two nodes adjacent to the failure bridge (copy) the working signals heading toward the failed span. The bridged copy is “looped back” to travel on protection capacity in the other direction around the ring. At the same time, the looped back signal arriving on protection capacity from the other adjacent node is looped back as a working signal. End-nodes continue to add and drop their signals to and from working capacity. This implementation is termed OCh shared protection with loopback switching. It resembles many of the features present in current conventional SONET bidirectional line switched rings, or terrestrial SDH shared protection rings.
OCh shared protection using end-node switching has a network advantage over protection using loopback switching. Transparent rings are engineered to support the longest possible optical path. For a ring of N nodes, the longest path for a working channel is N−1 spans. The longest path for a protected channel depends on the chosen switching mechanism. FIGS. 3A and 3B illustrate an eight node ring.
Under normal conditions, a working channel is added at node 1 (310), travels clockwise through seven spans, and is dropped at node 8 (380). Consider a situation where a cable is cut between nodes 7 (370) and 8 (380). When loopback switching is employed, the protected channel travels 13 spans, as shown in FIG. 3A. When end-node switching is employed, the protected channel travels 1 span, as shown in FIG. 3B.
For an OCh shared protection ring with N nodes, the worst case (protection) channel may travel through 2N−3 spans using loopback switching, or N−1 spans using end-node switching. Given equivalent node performances, a transparent ring employing loopback switching will be constrained to roughly half the size of a ring employing end-node switching.
Accordingly, there is a need to design a reconfigurable optical add-drop multiplexer that reduces node complexity with less optical devices in shared protection rings.